FIELD OF INVENTION
In the printing industry, and particularly in a printing process, a continuous web of paper is first passed through the printing press which makes ink impressions on the web. The moving web is then immediately passed through an oven to remove solvents and wetting solution retained from the printing process. The web is then cooled by passing it over chill rollers. The web may then pass through a sheet processing apparatus for cutting the web into separate sheets or signatures and alternately diverting or separating the individual sheets into two paths thereby creating a space between successive sheets. The continuous stream of spaced sheets are now in position for further processing.
The present invention relates to an improved system for decelerating and shingling or overlapping a stream of fast-moving, spaced apart sheets. The present invention employs a first deceleration portion which decelerates and reduces the gap between successive sheets and then further decelerates and shingles or overlaps the successive sheets for delivery of the sheets to a subsequent process such as a sheet counter or stacker system as described in my U.S. Pat. No. 4,652,197. The present invention provides an improved delivery system which maintains continuous, positive control over the sheets while the sheets are decelerated and also as they are shingled in preparation for delivery to a subsequent processing station. Critically, the present invention provides an improved delivery system which operates at a dramatically increased speed with respect to present delivery systems thereby allowing the overall printing operation to operate at an increased speed. The present invention operates equally well with signatures which are one sheet thick or with signatures which are several sheets thick such as pamphlets, magazines or newspapers.
Various delivery systems for decelerating and shingling sheets are set forth in the prior art. While printing presses operate at high speeds, it has always been necessary to reduce the speed of the sheets in the delivery system both to shingle and to square the sheets. Squaring the sheets may be achieved by allowing the lead edge of each sheet to strike a fixed object such as a squaring roller. However, to avoid permanent damage to the sheets, particularly single or lightweight sheets, the paper should not be travelling faster than about 300 feet per minute when squaring sheets. This limitation is a physical characteristic of most normal weight paper and, consequently, limits the overall output of the printing system by necessitating a reduced operating speed for the delivery system.
One delivery means known in the art operates in combination with opposed knife cylinders, each of which is 180.degree. out of phase with the knife edge of the other cylinder. These knife cylinders include a row of cam operated pins which pierce and grip the web and then deliver the cut sheet to an associated delivery cylinder. Each delivery cylinder includes a cam operated gripper that grabs the leading edge of the cut sheet as the pins in the knife cylinder are withdrawn and deposits the cut sheets in a shingled fashion on a delivery conveyor system. As each successive sheet is laid down in a shingled format on the respective delivery conveyors, the gripper of the delivery cylinder releases the sheet. However, operations such as these create a great deal of wasted paper because the sheet edges must be subsequently cut in order to remove the pin holes. Moreover, the need to cut the edges adds a further processing step to the overall system which increases the time to produce a finished product and increases the costs.
Another prior art delivery system employs a fan like element to shingle the sheets. By means of gravity, sheets are caused to fall into a receptive slot in a rotating fan-like delivery means. As the delivery means rotates the sheets fall out one after the other in an overlying or shingled arrangement. However, once a sheet has entered the fan delivery, the timing of the entire delivery system is subject to the gravitational forces working on the sheet. As a result, lightweight sheets could severely slow down a system otherwise capable of operating at higher speeds. The delivery system of the present invention improves upon this arrangement by maintaining continuous and positive control of each and every sheet, which this prior art system cannot do, and by increasing the operating speed with respect to this prior art system.
Other prior art delivery systems employ rotary knock down arms for decelerating the sheets but still require squaring rollers for aligning the sheets. While the knock down arms, by acting on the tail of the sheets, are an improvement over the use of fixed stops in decelerating the sheets, critical speed limitations are still present because of the squaring roller. Moreover, the knock down arm merely strikes the rapidly moving sheet throwing the sheet against a lower, slow speed belt. Because the sheet is unrestrained at this time the chance of it becoming misaligned or out of square is great.
An improvement over that system is disclosed in my U.S. Pat. No. 3,994,221. While still using a squaring roller, the deceleration procedure is improved by the use of a series of freely rotating snubber wheels mounted on rotating snubber support plates. Instead of only knocking the sheet down, allowing it to bounce onto a lower, slow speed belt, the snubber wheels physically trap the tails of the sheets against the slow speed belt while the lead edges of the sheets engage the squaring roller. This causes the sheets to decelerate more quickly but can still allow a misalignment. Consequently a squaring roller is still needed and still places a speed limitation on the system. However, this system is limited in its operating speed; at best, it can only handle a few pages of light weight stock travelling approximately 1800 feet per minute. Of course, heavier sheets can be handled at higher speeds.
The present invention overcomes all of the aforementioned problems by maintaining a positive control over the sheets during the decelerating process and during subsequent delivery. Specifically, as the sheets exit opposed, high-speed belts in a spaced format, a first slow-down section decelerates the sheets by employing snubber arcs to trap the individual sheets against lower, intermediate speed belts while the tail of the sheets are still engaged by or just exiting the opposed high-speed belts. While this may create a slight overfeed of the tail end of the sheets, it is not significant enough to permanently crease the sheets. This process reduces the gap between successive sheets and decelerates the sheets to the speed of the intermediate speed belts. The sheets are then delivered to a second decelerating process where snubbers trap the individual sheets against the lower, slow speed belts while the tails of the sheets are still engaged by the opposed intermediate speed belts. By decelerating the sheets in multiple slow down stages, the overall web speed can be increased and improve the speed of the printing operation. In addition, by maintaining positive control over the sheets, the sheets are never allowed to become unaligned. Thus, the continual positive control allows removal of squaring rollers which, in turn, allows the system to operate at a higher speed.